The power supply of the invention finds particular utility in external pedestrian crossing signals, or other types of signals, for maintaining the signals effective and visible over a defined range of ambient light levels. This is achieved by maintaining a constant contrast (brightness ratio) between the light level of the signal and the ambient light level. Such control of the light level of the signal is important because if the signal is too dim for a particular ambient light level it is difficult to discern, and if it is too bright it tends to cause glare.
It is, accordingly, a general objective of the present invention to provide a simple and inexpensive solid state regulated power supply for a luminescent lamp which enables the lamp to generate a variable light emission level in the presence of variations in ambient light levels so as to maintain a constant brightness ratio between the two light levels.